5 Steps to a 5 AP Chemistry 2019

196 ❯ STEP 4. Review the Knowledge You Need to Score High

4. Physical state of reactants—When reactants are mixed in the same physical
   state, the reaction rates should be higher than if they are in different states, be-
   cause there is a greater chance of collision. Also, gases and liquids tend to react
   faster than solids because of the increase in surface area. The more chance for
   collision, the faster the reaction rate.


5. Catalysts—A catalyst is a substance that speeds up the reaction rate and is (at
   least theoretically) recoverable at the end of the reaction in an unchanged form.
   Catalysts accomplish this by reducing the activation energy of the reaction.
   Activation energy is that minimum amount of energy that must be supplied to
   the reactants in order to initiate or start the reaction. Many times the activation
   energy is supplied by the kinetic energy of the reactants.

Keywords and Equations

ln[A]t - ln[A] 0 = – kt (first order)

1

[A]

1

[A] 0

−=kt

t

(second order)

In 20.693

t1/2==kk

In

1

a In A











= 

−

k +

E

RT

t = time (seconds)

Ea = activation energy

k = rate constant

A = frequency factor

Gas constant, R = 8.314/J mol-^1 K-^1

Rates of Reaction

The rate (or speed) of reaction is related to the change in concentration of either a reactant or

product with time. Consider the general reaction: 2A + B → C + 3D. As the reaction proceeds,

the concentrations of reactants A and B will decrease and the concentrations of products

C and D will increase. Thus, the rate can be expressed in the following ways:

tttt

Rate

1

2

[A][B] [C]1

3

[D]

=−

∆

∆

=−

∆

∆

=

∆

∆

=

∆

∆

The first two expressions involving the reactants are negative, because their concentra-

tions will decrease with time. The square brackets represent moles per liter concentration

(molarity).

The rate of reaction decreases during the course of the reaction. The rate that is cal-

culated above can be expressed as the average rate of reaction over a given time frame or,

more commonly, as the initial reaction rate—the rate of reaction at the instant the reactants

are mixed.

KEY IDEA